Aviation gasoline production



Patented July A9, Y z;

VRoberti?. Marschner, Homewood,l Ill., assignor toStandard Oil CompanmnChicago, Ill., a cor,-

V poration of ,Indiana` Application April 29, 1942, Serial No 440,989

solaires. y(c1. '26o-6831 This invention relates to aviation gasoline production and it pertains more` particularly to improved methods and means for processing amixture'of hydrocarbon gases containing normal butane, isobutane, propane, butene-butene-l, isobutylene and propylene.

Heretofore refinery gas streams containingvCa and C4, olenic. hydrocarbons have been thermally or catalytically polymerized to produce nonselective polymers (chiefly dimers and trimers with more or less intermediate and heavier products) which are characterized bya relatively wide and rather high boiling-range and an octane num.-

ber in the general vicinity of r15430. 'Such polymer is suitable for ordinary motor fuel but it is not suitable, because oi its highunsaturation, for aviation gasoline. On hydrogenationito `remove the unsaturation the octane number of such nonselective polymer is not improved to any appreciable extent and 1in some cases itis actually lowered. When the charge contains large amounts of propylene and only small amounts of butylenes there is a tendency toward the production of the propylene trimer,'a mixture 'of C9 olens which have an average octane number yof about 80 but which upon hydrogenation give paraflins havingan octane number of only about 60-70. Hydrogenated non-selective lpolymer is, therefore, unsuitable for aviation gasoline not only because of its volatility `characteristics but because of the low octane number of the hydrogenated polymers. Since both low volatility and low octane number appear to be associated with propylene, thisf'olen is customarily considered worthless as a raw material for aviation gasoline. An object of my invention is to provide an improved. method and means for producing a hydrogenated copolymer of about C6 toCs (chiefly Cv) boiling range which is characterized by a high octane number.

Selective polymerization of butenes results in a dimer or codimer having an voctane number of approximately 82 to 84 and on hydrogenation the hydrodimer or hydrocodimer has an octane number of about 92 to 94. The'term hydrodimer is hereby deiined `to vmean hydrogenated dimer and the Word hydrocodimer-'is deiined to mean hydrogenated codimer. In'such a process, however, there is no utilization of the propylene content of the charging stock and two mols of C4 olen are required to produce one mol of aviation gasoline. Furthermore, the resulting isooctane product is characterized by a vrelatively narrow boiling range which is higher than that desired for aviation gasoline. Most light,` hydrocarbon mixtures with which such isooctanes can beblendedhave either too lowan octane number or too high avapor pressure or both for aviation gasoline. An object of my invention is to decreasey the amountgof Cri-C6 hydrocarbons requiredfor the blending with isooctanesv and to provide a C'r hydrocarbon blending stock for simultaneously providing optimum volatility and maximum octane number. A further object is to increase the yield of avia-tion gasoline'y per unit lof, C4 oleflns available.Y A Y The Ca-Ci refinery streams which contain isobutane as well as oleiins have been chargedto an alkylation4 process for the direct production o f isoparaiiins of the aviationrgasoline boiling range. In, alkylation processesl the propylene-isobutane alkylate has*` a much lowerA octane number vand consumes much greater quantities Vof, catalyst than the butene-isobutane. alkylate.` Furthermorer `,the isobutylene-isobutane alkylate has a lower octane number than the'normal buteneisobutane alkylate rand generally speaking the presence of isobutylene 'in the alkylation charge results in greater acid. (catalyst) requirements and less desirable yields and product distribution. It has, therefore, 'been proposed to subject a normal butene-isobutylene mixture rst to selective polymerization for utilizing the isobutylene com-f ponent and then to alkylate the remaining butenes with isobutane. In such processes a considerable amount of the butene-2 is used up in the .polymerization step and 'is hence unavailable for alkylation. Here again the propylene content of the charging stock is not utilized. An important object ofA my invention is to utilize the propylene and isobutylene ,content 0f the charging .stock most effectively in a polymerization step andto conserve the butene-2 component of` the charge for subsequent alkylation with isobutane.

A further object of my invention is .to provide "a unitary system oi polymerization, fractionation and hydrogenation which may be supplemented by alkylation and which may/be further supplemented by butane isomerization in order to produce maximum yields of a superior aviation-gasoline. Other objects will be apparent as the detailed description lof the 'invention proceeds.

In my preferred operation I initially fractionate the Ca-Cihydrocarbon charging stock mixture between normal butane and butene-l; at atmospheric pressures this'would mean fraction'- ation at about 25 F. The overhead fraction will thus contain propylene (-54 FJ, propane (-44 FJ, isobutane (10.9" FJ, isobutylene (1gb FJ, and butene-1 (20 E). The bottoms fraction from this fractionation step Will consist essentially of normal butane (31 F.) and butene-2 (34 lik-38 F.). The overhead fraction which contains the isobutylene and propylene but which does not contain substantial amounts of butene-2, is charged to a polymerization step employing a catalyst such as copper pyrophosphate, phosphoric acid or any other known polymerization catalyst in order to accelerate the polymerization reaction. In this step the isobutylene and propylene are utilized to the maximum practicable extent for the formation of polymer and the conditions of the polymerization are regulated to produce a maximum of heptenes and a minimum of nonenes and heavier hydrocarbons. The heptene polymer fraction, which may also contain Ce and C8 olefins, is `then separated from the C9 and heavier olefins,'the latter being utilized in -motor gasoline. The heptene polymer fraction is hydrogenated and is thus converted into a valuable aviation gasoline fraction.

The normally gaseous hydrocarbons recovered by debutanization of the polymerization products are fractionated to separate propane and any residual propylene from isobutane and butene-2. This fractionation at atmospheric pressure would be at about F. or lower. The overhead gas is discarded and the isobutane and butene-2 is commingled with the normal butane-butene-2 fraction from the original fractionation step for alkylation with sulfuric acid or other known alkylation catalyst. The normal butane from the alkylation products may be isomerized to isobutane and returned to the alkylation step. 'Ihe resulting alkylate may then be blended with the hydrogenated heptenes for the final aviation gasoline blend.

By removing the butene-2 from the charge to the polymerization step I minimize the formation of Ca copolymer and obtain a maximum amount of C7 hydrocarbons and a product which when hydrogenated has a very high antiknock rating. Some of the butene-1 will polymerize with isobutylene but this polymerization does not proceed quite as rapidly or to the same extent as the corresponding polymerization of isobutylene with butene-2 or of isobutylene with propylene. The butene off-gases from the polymerization step are almost entirely butene-2 (evidently the butene-1 is isomerized to butene-2 in the polymerization step). Some of this butene-l (and its butene-2 isomer) is used up in the polymerization step but a substantial amount of the butene-2 is recovered with the isobutane for alkylation with the butene-2 which has by-passed the polymerization step. The depropanized off-gas is thus by itself an almost ideal alkylation feed, containing no expensive low-yield, low octane number olens and no diluent n-butane, but in volume it represents only a portion of the potential raw material for aviation gasoline. Thus by alkylating the polymerization off-gas and blending the alkylate with the approximately equal volume of hydrogenated fractionated codimer ordinarily obtained, a nished aviation gasoline results but by means of alkylation of the isomerized and alkylated n-butane-butene-Z cut a much larger amountof high octane number alkylate results,

and even though the blend with the hydrocodimer no longer passes aviation gasoline volatility specifications, this operation is ordinarily advisable.

The invention will be more clearly understood from the following detailed description read 111 conjunction with the accompanying drawing which forms a part of this specification and which is a schematic flow diagram of my improved system for convertingv C3-C4 refinery gases into maximum yields of maximum quality aviation gasoline.

The charging stock from my system may be a mixture of gases from any source containing propylene, iso and normal butenes and isobutane. The latter may be obtained by isomerizing normal butane if it is not present in sufficient amounts in the charging stock. Such a gas mixture is produced in practically all thermal and catalytic cracking, reforming and gas reversion processes. I prefer a mixture in which the isobutylene content is approximately as great or greater than the propylene content but my invention is not limited to any particular charging stock composition.

As a specic example of my invention I will describe the use of a gas mixture obtained from a thermal or catalytic cracking process. The invention is particularly adaptable to gases produced by catalytic cracking of normally liquid hydrocarbons, in which gases isobutane is more abundant and the polymerization step feed therefore represents a larger proportion` of the total gas. 'I'he light products from this thermal or catalytic process may be passed through a high pressure separator for the removal of noncondensible gases such as hydrogen, methane and most of the ethane and ethylene. The remaining cracked gasoline plus C3 and C4 hydrocarbons may be introduced by line I0 into gasoline stabilizer II. The gasoline fraction may be Withdrawn directly through line I2 to motor gasoline storage tank I 3,

The mixture of C3 and C4 hydrocarbons passes from the top of the stabilizer through line I4 to gas fractionator I5 which may be a single column or a plurality of columns 4for separating hydrocarbons boiling above about 25 F. from hydrocarbons boiling below 25 F. rI'hus fractionating column I5 may be designed to separate C3 hydrocarbons from Cl hydrocarbons, the propane and propylene passing overhead through line I6 and the butanes and butylenes being withdrawn from the bottom through line I1. to a second gas fractionating column I 8. If desired, a minor portion of propane and propylene may be discarded through line IGa in order to maintain the optimum propylene to isobutene ratio. The second gas fractionating column I8 is designed and operated so that isobutane, isobutylene and butene- 1 are taken overhead through line I9 while normal butane and butene-2 are withdrawn from the base through line 2U.

The'propane and propylene from line I6 together with isobutane, isobutylene and butene-1 from line I9 are introduced through line 2I to polymerization reactor 22. The polymerization may be effected with a copper pyrophcsphate catalyst at a temperature within the approximate range of about 300 to 500 F. for example within the general vicinity of about 400 F. under a pressure of about 500 to 1500 pounds per square inch, for example about 1200 pounds per square inch. The flow rate through the reactor will depend on the concentration of the olefins in the charging stock and will usually range from approximately 10 to 50 cubic feet per hour (mea:- ured at 60 F. and 1 atmosphere) per pound or" catalyst in the reactor. The copper pyrophosphate catalyst may be prepared from stoichiometric proportions of reactants to obtain CuzPzOv 5 orI mayemploy an excess of sodium pyrophos- :phate'or copper sulfate. This catalyst is preferablyfsupported on a carrier such as highly porous carbon or other known catalyst support. Where a phosphoric acid catalyst is employed it is usually supported on kieselguhr and the polymerization is effected within the approximate temperature range of 300 to 480 F. under a pressure ized or otherwise pretreated, that a plurality of *Y k polymerization reactors may be employed in series or in parallel either concurrently or countercurrently and that all known expedients may be employed `for the effecting of theV desired polymerization. For example, the conversion may be reduced and the entire off-gas may be recycled through the reactor together with fresh feed to force the reaction of isobutylene with other olefins rather than with itself. Similarly, the depropanized overhead may be recycled through the polymerization to increase prcductionof heptenes. The conversion in the polymerization reaction is preferably Within the range 50 to 75%, depending upon the relative amounts of isobutylene, butene-l and propylene present. With typical thermally cracked gases, the isobutylene conversion is preferably'above 90% and the propylene conversion can be maintained above v80% without a correspondingly large clean-up of the normal butenes.

The `polymerization products 'from reactor or polymerization system 22 are introduced'through line 23 to debutanizer tower 24 which is designed and operated so that gases including C3 and C4 hydrocarbons are taken overhead through line 25 andnormallyliduid polymers are removed from the bottom through line 26. rIhis polymer iraction is usually of relatively wide boiling range and while it is a very good blending stock for motor gasoline it is not yet a satisfactory aviation gasoline. If the entire copolymer is hydrogenated the resulting octane number is ordinarily not materially increased. I have discovered however that if the light fraction of this polymer, consisting essentially of Cra-Cv and some vCa hydrocarbons, is separated from the C9 and heavier hydrocarbons, the hydrogenation step will effect a material increase in the octane number. I introduce the polymer from line 26 to fractionator within the approximate range of 300 to 3000 pounds or more per square inch, the temperature may bevwithin the approximate range of 550 to 850 F. and the space velocity may be approximately 1 to 5 volumes of charging stock (liquid basis) per hour per volume of catalyst space in the reactor.v Alternatively I may employ a catalyst such as copper, cobalt or preferably nickel or the lower oxides of such metals supported on pumiceysilica or the like. If catalysts such as nicke1 or nickel oxides are employed the hydrogenation may be effected at pressures ranging from near atmospheric to about 50 pounds or more per square inch, the temperatures may be within the approximate range of 350 to 450 F. land the space velocities of the orderof 2 to 10 lvolumes (liquid basis) of charging stockper hour per Volume of y'catalyst space. No invention is 'claimed in the hydrogenation step per se vand hydrogenation system 29-is intended to include all known features and expedients which may be necessary or 'desirable in such system. The hydrogenated C7 hydrocarbon polymer together with the Cs and Cs hydrocarbons associated therewith are withdrawn from the hydrogenation system through line 30 and introduced into aviation gasoline storage tank 3|. K

' The Vdebutanizer gases fromthe top of tower 24 are introduced through line 25 to gas fractionation column 32 which is designed'r and 0p'- Y y erated to remove propane, propylene 'and any lighter gases as an overhead throughline 33a. The isobutane and butene-2 is withdrawn from the base of fractionator 32 through line 33 and v'2l which is designed and operated to take overe head through line 28 the Cs, C7 and C8 polymers,

i. e., all liquids boiling up tol about 200 to 250 F.,

for example up to 230 F. V

This light fraction is-then passed to a hydrogenation system 29 wherein it is hydrogenated with the aid ofrany known type of hydrogenation catalyst to eiect saturation ofthe light ypolymers, without material production of lighter hydrocarbons such as methane or butane. As catalyst,.I may use oxides or sulfides of group VI metals such as molybdenum, chromium, tungsten, etc., eitheralone or in admixture with each other or with other metallic oxides, and I may mount such catalysts on suitable supports such as Activated Alumina', alumina gel, bauxite, etc. With 'such catalysts thel operating pressure maybe this stream together with the normal butanebutene-Z stream from line 20 are introduced by line 3i!` toY alkylation system 35. This' alkylation system may be ofthe sulfuric acid typeV employing inlet acid concentrations of about 96 to 98%and outlet titratable acids in therange of about 88 to l92% or a similar process in which the acid concentration is maintained constant at 92-94%. Y The acid requirements for this system are minimized by the elimination of the propylene and isobutylene from the charging stock and by maintaining a relativelyhigh isobutane to butene-2 ratio. The elimination of the isobutylene and butene-l from the alkylation charging stock also results in higher yields and products of higher octane number than would otherwise be obtainable.V The alkylation maybe effected in high speed mixers for insuring intimate contact and with cooling means for maintaining 'the alkylation temperature within the trange of about 30 to'O"V F. Approximately 0.3 to' 0.4 volurneof fresh acid and about 2.5 to 3l volumes of `recycled acid may be charged to the reactor for each volume of liquid charging stock introduced y thereto.- No invention is claimed in the alkyla tion step per se sincey the method and conditions for operating'such processes are well known to those skilled in the art and alkylation ,system'35 is intended to include all expedients known to the art for such operation. My invention is not limited to thefuse of a sulfuric acid alkylation system since other catalysts such as aluminum chloride, metal oxides, etc. are vwell known to those` skilled in the art.l

'The alkylation products are introduced by line 36 to debutanizer column 31,'from the base of which the alkylate is withdrawn through line 38' and introduced' into aviation gasoline storage tank 3! Unreacted gases may be recycled to the alkylation step. The debutanizer gases leaving the top of tower 31/may be introduced through Aline 39 to a butane isomerization system 40 for converting normal butane into isobutane and the resulting isobutane may be returned by line 4| and line 34 to the alkylation system 35. If desired the debutanizer gases from line 39 may first be fractionated so that only the normal butane fraction is passed to the isomerization system and the unreacted isobutane is recycled directly to the alkylation system. The butane isomerization system may employ a catalyst consisting essentially of aluminum chloride distributed on a porous support of solid inorganic material Which has been partially but not completely dehydrated or on clays of the type employed for the refining of animal, mineral or vegetable oils. Alternatively, the catalyst may be an aluminum chloride-paraflinic hydrocarbon complex in liquid form or may be aluminum chloride dissolved in a diluent or in the liquid charging stock itself. Hydrogen chloride is usually employed for promoting this isomerization reaction and it may be effected at temperatures ranging from room temperature to 200 to 300 F. No invention is claimed in the isomerization step per se, and since this process is Well known in the art it Will not be described in further detail.

The aviation gasoline properties of the blended gasoline in tank 3! may be adjusted by the addition of other hydrocarbons. Isopentane may be added to the extent of 5-10% to further increase the volatility, the amount necessary to give 7 pounds Ried vapor pressure being only about half that required by isooctane. The boiling range may be lowered Without decreasing the octane number by the addition of up to 40% light parafiins such as neohexane or light naphthenes such as cyclopentane. The volume of aviation gasoline may be considerably increased at some expense in octane number by dilution with -40% straight-run naphthas, and may be greatly increased by blending with nearly an equal amount isomate or catalytically cracked naphthas. Isomate is the normally liquid product fraction in the gasoline boiling range resulting from the isomerization of light naphtha with an aluminum `chloride-hydrocarbon complex or other halide isomerization catalyst. The combustion characteristics may be modified iby the addition of 5-40% aromatics. In many cases, a combination of these adjustments Will be desirable to t the particular circumstances of demand and supply at any particular refinery. In any case either a greater volume of a standard grade of aviation gasoline, or a superior grade of the same amount of aviation gasoline results by replacing isooctane 'by my hydrocodimeralkylate blend.

From the above description it Will be seen that I have attained the objects of my invention. My aviation gasoline contains large amounts of parafnic C7 hydrocarbons of high octane number as well as octanes obtained by alkylation so that this gasoline is characterized not only :by remarkably high octane number but also by optimum volatility characteristics. I have obtained maximum utilization of propylene and I have utilized the butenes more effectively than they have ever before been utilized for the preparationvof aviation gasoline. Approximately one mol of aviation gasoline is pro-duced for every mol of C4 olen available in the feed. Acid requirements have been minimized in the alkylation step and the alkylation products have been improved both as to yield and as to octane number. Hydrogenation has been limited to a specific polymer fraction wherein its use is markedly benecial and I have thereby rendered this specic light fraction of polymer suitable for use in aviation gasoline.

While I have described a particular embodiment of my invention it should be understood that the invention is not limited tothis specific example or to the recited details of operation since many alternative and equivalent systems will be apparent to those skilled in the art from the above detailed description.

I claim:

1. The method of obtaining maximum utilization of a hydrocarbon gas stream containing propylene, isobutylene, and normal butenes for the preparation of aviation gasoline which method comprises fractionating said gas stream into a light fraction containing predominately propylene, butene-l, and isobutylene and a heavy fraction containing butene-2, polymerizing the light fraction under conditions for effecting the formation of substantial amounts of a heptene fraction, hydrogenating said heptenel fraction under conditions for effecting saturation, alkylating the heavy fraction containing butene-2 with isobutane to form alkylate isooctane and blending said alkylate isooctane with said hydrogenated heptene fraction.

2. The method of claim 1 wherein the light fraction contains isobutane and which includes the further step of separating a gas fraction containing isobutane and butene-Z from the products of the polymerization step and introducing said gas fraction into said alkylation step.

3. The method of claim 1 wherein the heavy fraction contains normal butane which method includes the further step of isomerizing said normal -b-utane to isobutane and introducing said isobutane into said aikylation step.

4. The method of claim 1 which includes the further step of remo-ving heavy polymers from said heptene fraction prior to said hydrogenation step.

5. The method of making an aviation gasoline from a gas stream containing propylene, isobutylene, normal butenes, isobutane and normal butane which method comprises fractionating said gas stream into a light fraction containing propylene, isobutylene, butene-l, and isobutane and a heavy fraction containing butene-Z and normal butane, polymerizing said light fraction under conditions for producing substantial yields of C7 polymers, separating said polymers from unreacted gases, hydrogenating at least the C7 fraction of said separated polymers, separating an isobutane-butene-Z fraction from the separated unreacted polymerization gases, combining said separated isobutane-butene-Z fraction with said heavy fraction and allcylating the resulting mixture to form alkylate isooctane, isomerizing unreacted normal butane to form isobutane, introducing said isobutane into said alkylation step and blending at least a part of said alkylate isooctane with at least a part of said hydrogenated polymer.

6. The method of making aviation gasoline from a hydrocarbon gas stream containing propylene, isobutylene, normal butenes, isobutane and normal butane which method comprises fractionating said gas stream into a rst overhead fraction containing propylene and a rst bottom fraction containing butanes and butenes, fractionating said rst lbottom fraction to obtain a second overhead containing isobutylene, butene-l and isobutane and a second bottom fraction containing butene-2 and normal butane, combining said iirst oyerhead fraction with said second overhead fraction and polymerizing said combined fractions 'under conditions for obtaining heptenes, separating a fraction 5 containingv said heptenes and hydrogenating said fraction, separating a gas stream containing isobutane and butene-2 from polymerization off- 

